Frequency modulation, nuclear magnetic

Rotational motion of a nitroxide modulates the anisotropic electron-nuclear magnetic dipolar interaction, giving rise to electron relaxation that affects the EPR spectral line shape. At X-band frequency, the spectra are sensitive to motions with correlation times in the range of 10-11 < rc < 10-7, but also reflect the anisotropy of the motion. Figure 5B shows simulated EPR spectra for isotropic motion in the fast, intermediate, and slow motional regimes and illustrates the high sensitivity of the line shape to motional rate. 

Electron spin resonance (ESR) spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. There is a variety of ESR techniques, each with its own advantages. In continuous wave ESR (CW-ESR), the sample is subjected to a continuous beam of microwave irradiation of fixed frequency and the magnetic field is swept. Different microwave frequencies may be used and they are denoted as S-band (3.5 GHz),X-band (9.25 GHz), K-band (20 GHz), Q-band (35 GHz) and W-band (95 GHz). Other techniques, such as electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, record in essence the NMR spectra of paramagnetic species. 

P.K. Madhu, A. Goldbourt, L. Frydman, S. Vega, Fast radio-frequency amplitude modulation in mrdtiple-quantum magic-angle-spinning nuclear magnetic resonance theory and experiments, J. Chem. Phys. 112 (2000) 2377—2391. 

S. V. Dvinskikh, H. Zimmermaim, A. Maliniak, and D. Sandstrom, Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-, phase-, and frequency-modulated Lee-Goldburg cross-polarization, J. Chem. Phys. 122 044512 (2005). 

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